And the answer is"..5.57 inches.
A new study, completed at Indiana University and published in the Journal of Sexual Medicine, reviewed the length of the penis of 1,661 sexually active men to determine the average size.
A Wide Range of Penis Sizes
Researchers requested the size of an erect penis in order to give properly fitted condoms. Men were told that reporting the wrong size would result in having the condom either baggy (if they reported a larger penis size) or too small (if they reported a smaller size.) The lead author, Debby Herbenick, believes that most men reported the size of their erect penis accurately.
According to the numbers reported, the average erect penis is 14.15 cm (5.57 inches) long with a circumference of 12.23 cm (4.81 inches). But, Herbenick states there was a "broad range of penile sizes" ranging from 4 to 26 cm long (1.57 inches to 10.24 inches). 
An interesting discovery in the research was that the size of an erect penis varied based on how a man was aroused. "For example, men who received oral sex as part of the measurement process reported a greater erect length and circumference than did some other men." But Herbenick isn't sure why this happens, "Is that because oral sex is more arousing to men and thus they have larger erections from it than they do from other sexual behaviors (e.g. self-masturbation with hand or hand stimulation from a partner or fantasy only, etc)? Or are men who have a larger erect penis more likely to receive oral sex from a partner (or more regularly)? We cannot tell from these data." 
Does Size Really Matter?
For women, size, even if it matters, isn't usually very high on the priority list. Intimacy, affection and emotional connection are more important than the size of your partner's genitals. But to men, their size represents their masculinity - and there is a constant fear that their penis is smaller than anyone else's.
Some believe it is a matter of viewing perspective. When men look at their own penis, they are looking down and only seeing it from one angle - the one that makes it look smaller. But when they check out the size of other men (in men's rooms and locker rooms) they are seeing it from the side, in other words, seeing the entire length of it, and the view of their own rarely lives up to that view. Many men believe that it would be better "if it was just a little bigger," despite reminders from partners that size really doesn't matter.
And while there are many companies that promise to enlarge your penis, there isn't any proven way to do so without surgery. But there are ways to improve your lovemaking, no matter what the size of your penis. Herbenick suggests, "Just try talking with your partner to share more about what you'd like to try, how you want to connect, and what you crave in terms of intimacy. Ask your partner what he or she wants and needs from you too."  She also suggests books about sex, attending sex-positive workshops and sex counseling or therapy.
"Erect Penile Length and Circumference Dimensions of 1,661 Sexually Active Men in the United States," 2013, July 10, Debby Herbenick, Ph.D., M.P.H. et al, The Journal of Sexual Medcine
   "Our New Research on the Penis Sizes of 1,661 American Men," 2013, July 19, Debby Herbenick, Ph.D., M.P.H., Psychology Today: The Pleasures of Sex Blog